In vehicles, especially, those equipped with diesel engines, an exhaust gas recirculation system is most frequently incorporated for recirculation of a portion of engine exhaust gases as a part of new charge therely minimizing the amount of oxides of nitrogen contained in the exhaust gases. However, when such an exhaust gas recirculation system is used, solid carbon contained in the exhaust gases is also mixed into intake air, and the carbon tends to be contained in the engine oil. There has therefore been the tendency that articles of carbon flow into the area between, for example, cams arranged for rotation for causing opening and closing movement of inlet valves and exhaust valves of the engine and slipper faces of valve rocker arms swung by the cams, resulting in unusual abrasion of the slipper faces having a hardness lower than that of the surface of the cams.
To avoid such a trouble, formation of this slipper portion from a ceramic material this kind of silicon nitride is disclosed in, for example, Japanese Patent Laid Open No. 217307/1984. The structure disclosed in the cited patent application is shown in FIG. 1. Referring to FIG. 1, a valve rocker arm body 102 is rotatably or rockably mounted on a rocker arm shaft 101. One end of the valve rocker arm body 102 is engaged by the upper end of a valve stem 103 of an inlet valve or an exhaust valve in an engine. The valve stem 103 is normally biased upward by the force of a valve spring 104 through a spring retainer 105. A cam 106 driven from the engine engages the other or base end of the valve rocker arm body 102 for causing rocking movement of the valve rocker arm body 102 around the shaft 101. A ceramic tip 108 having a slipper face 107 actually engaged by the cam 106 is provided integrally with the base end of the valve rocker arm body 102. Generally, this ceramic tip 108 is cast integrally in the base end of the valve rocker arm body 102 during casting of the valve rocker arm body 102.
As shown in FIG. 2 which is an enlarged perspective external view of the prior art ceramic tip 108, a pair of wedge-like tapered retaining faces 110 are formed on a cast body block 109 of the ceramic tip 108 for preventing escapement of the ceramic tip 108 from the valve rocker arm body 102. However, when, in the ceramic tip 108 having such a structure, an impulsive force generated due to the rotation of the cam 106 is repeatedly imparted, as moment of rotation, to a slipper block 111 having the slipper face 107, cracks tend to occur at the constricted basal part of the cast body block 109. Thus, the prior art ceramic tip has been shown to be defective in that the possibility of damage due to generation of cracks is very large resulting in a very low reliability.